Faba beans are a poor competitor, so good weed control is essential and often the most important consideration for profitable faba bean production. It may be beneficial to utilize an integrated weed management strategy, combining both herbicides and cultural control methods.
The major weeds of concern in faba bean crops are Canada thistle and perennial sow thistle. Other significant weeds include: cow cockle, roundleaved mallow, bluebur, cleavers, and wild tomato.
Weed Seedling Identification
Good weed management begins with knowing what weeds are present. Perennial weeds are best controlled pre-harvest in the previous crop. Fields should be inspected again before freeze-up, first thing in the spring as this is an ideal time to control winter annuals. The next weed inspection should be just prior to planting to time pre-seed burn-off. Fields should be inspected again upon crop emergence to establish the frequency and distribution of weed species and to determine appropriate herbicide products for post-emergent weed control.
Herbicide Resistant Weeds
Weed populations contain a great deal of natural variability to survive changing environmental conditions. Herbicides select for individual plants with the ability to survive the herbicide treatment. Surviving plants then contribute to a larger proportion of the weed population. It is important to realize that selection for herbicide resistant weeds occurs each time an herbicide is applied.
Herbicides have different modes of action. Some modes of action are easy for weeds to develop resistance to as it only requires variation in a few genes (high risk of resistance), while others may require changes in multiple genes (low risk of resistance). Weeds that have developed resistance in the Prairies are listed at: http://www.weedscience.org.
Table 5. Herbicides Registered for pre-seed or pre-emergence applications in Faba Bean (as of April 1, 2017)
Note: Check product labels for herbicide group number(s) to confirm the mode of action as new products and mixtures are always being introduced.
Group 1 and 2 herbicides have a high risk of developing resistant weed populations, therefore, management of these modes of action is important to lengthen the time they are able to provide effective control. If a grower has applied Group 1 or 2 herbicides more than 10 times in a field without a second effective mode of action, there is a high risk of resistance developing among one or more weed species.
Before applying herbicides consider the field’s herbicide history and consider tankmixing modes of action or using herbicide group rotation to avoid the development of herbicide-resistant weeds.
Herbicide-resistant weeds are more likely to occur under the following conditions:
- High weed number
- Too frequent use of a single herbicide group or mode of action
- Not using recommended rates
- Allowing surviving weeds to set seed
Resistance is not reversible. That means stopping the use of the herbicide will not change the frequency of resistance in the weed population. However, research indicates that alternating between two modes of action for wild oat control will double the number of years for resistance build-up, and alternating with a third mode of action will quadruple the time of resistance build-up (Cavan et al., 2001).
The decision to spray or not to spray should be based on economics. If the potential yield loss is greater than the cost of the chemical and application, then you should spray. Consult the current Saskatchewan Ministry of Agriculture's Guide to Crop Protection for examples of economic thresholds and situations where the choice to spray or re-spray may not be clear.
Prior to spraying, producers should consider the following spray tips:
- Evaluate sensitivity of the surrounding environment and avoid spray drift
- Inform your neighbors about the location of the faba bean crop
- During periods of crop stress (heat, drought, frost, or after land rolling), the ability of the faba bean crop to tolerate herbicides may be reduced. Crop injury can be reduced by:
- waiting approximately four days after the crop stress occurs before applying a herbicide
- maintaining water volumes at label recommendations
- Adhere to label directions for application (ie: water volumes, crop and weed staging, weather conditions etc)
- Thoroughly clean sprayers according to the label directions of the previous herbicide, as very small amounts of residue of some herbicides can cause significant crop injury
- Ensure your sprayer is functioning properly (leaks, plugs, pressure gauge, etc.) and choose appropriate nozzles for the situation
Always follow label recommendations and check product labels carefully. Your most up-to-date label information is available on the manufacturer’s web site links to the product label.
Cultural Weed Control
There are several methods of cultural weed control:
- Use clean, weed-free seed
- Use diverse crop rotations
- Good sanitation practices, such as cleaning harvest and seeding equipment to avoid spreading weed seeds between fields
- Seed early. Faba bean plants are more competitive with annual weeds if they emerge rapidly and cover the soil surface before the weeds emerge
- Spring tillage, even minor tillage, significantly increases the burial and resulting germination of false cleavers and catchweed bedstraw. However, growers should limit spring tillage as part of an integrated weed management program (Reid et al., 2005).
- Make the crop more competitive (i.e. higher seeding rates, appropriate fertility levels) to help choke out weeds and help reduce the reliance on herbicides